Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltates

We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-\delta}$. By using the relation between the temperature, partial oxygen pressure and the oxygen content from the thermogravimetric analysis, we have to synthesized a series of samples with precisely controlled $\delta=0.00-0.49$. The samples show significant coupling among the structural, magnetic and transport properties as a function of $\delta$. The stoichiometric material with $\delta=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_C=274$ K. The increase of $\delta$ to 0.15 is followed by a linear decrease of $T_C$ to $\approx$ 160 K and a metal- insulator transition within the cubic structure range. Further increase of $\delta$ results in formation of orthorhombic $a_p\times a_p \times 2a_p$ phase for $\delta\approx 0.25$ and brownmillerite phase for $\delta\approx 0.5$. Those phases are weak ferromagnetic insulators with $T_C=230$ K and 120 K, respectively. The present data show that the control of oxygen stoichiometry in lanthanum strontium cobaltates allows to modify the crystal structure and physical properties of these materials.